3. SITE SELECTION
Biopiles require adequate space for equipment access, soil hauling, soil storage, and availablility of utilities such as water and power. Local, remote, and temporary or permanent treatment sites are frequently dictated by the amount of soil to be treated (>250 cy).
4. SOIL PREPARATION
Tilling, discing, shredding, and/or screening may be necessary to reduce particle size and provide a more homogeneous matrix. Blending or mixing agents such as sand, wood chips, straw, mulch, grass clippings, compost, gypsum, waste from grape producers (shredded stems, pumice, etc.) may be added to increase soil porosity, homogeneity, and/or retain moisture. Reducing particle size prior to excavation by plowing, cultivating, or discing may be all that is required for select soils. Typical biopile blending agent additions range from 10 to 20% combined with 80 to 90% contaminated soil.
5. BIOPILE CONTROL PARAMETERS
• Moisture – 75% field holding capacity (12 to 30% by weight)
• Biochemical Catalysts –Typical C:N:P ratio 100:10:1 to 100:1:0.5 depending oncontaminants and degraders
• Excessive concentrations of phosphate and sulfate can retard microbial metabolism
• Oxygen Concentration – mechanical aeration is superior to chemical oxidization
• Hydrogen Peroxide wastes oxidization on natural organics in soil that are not target contaminants
• pH – 5 to 9, adjust with soda ash, NaOH, citric acid
• Mass Transfer Confirmation – radius of influence testing by air injection and magnehillic gages
• Soil Temperature – 10ºC to 45ºC (50 to 113ºF). For every 10ºF rise in temperature, degradation rates may double.
• Regenerative Blowers - produce heat in the range of 140ºF which is ideal for pressurized aeration of biopiles.
6. BENCH-SCALE TESTS
A standard method for biopile treatability studies does not exist. Bench-scale treatability tests do not provide reliable scale up information for full-scale design. Catalyst and carbon requirements are dynamic and are different for each soil and contaminant. Maintaining elevated heterotrophic plate counts after amendment addition is a good indicator that degradation is proceeding, and adequate moisture levels are being maintained.
Amendment frequencies and dosages may need to be adjusted to achieve optimal degradation rates. Note that as petroleum hydrocarbon chains are degraded or broken down into smaller more numerous chains, a “bouncing ball” degradation curve may be observed, with concentrations dropping and rebounding until asymptotic concentrations are reached. Applying too much organic catalysts can be detrimental as microbes may prefer to degrade additives rather than target contaminants.
Remtech has performed soil bench-scale tests using metal (minimal contaminant adsorption) pans or pails. Prior to splitting soil between different bioreactors, rocks and roots are removed, clumps broken down with nitrile gloves, then screened. Catalysts are added with spray bottles that deliver a known amount per pump, mixing with a stainless-steel paddle or spoon, and adding distilled water to maintain adequate moisture without producing a pasty soil with no free liquids in the bottom of the container. Additional water may be required to ensure that catalysts are distributed throughout the entire matrix. Samples can be removed from the same test vessel providing mixing is maintained during the test period to determine degradation rates and plate counts. A contaminated sample blank should be run without catalyst addition for natural attenuation comparison.
Continuously Stirred Slurry Reactors (CSTRs) with shakers or magnetic stirrers have also been used by Remtech to determine relative biodegradation rates using erlenmeyer flasks covered with tin foil. Flask studies are primarily used for water-phased bioremediation studies, not biopile studies.
7. BIOPILE CONSTRUCTION
Select biopile configurations are presented in Figures 2 and 3. Adding or eliminating system options may be appropriate for specific contaminates and soils. General construction rules thumb are summarized below:
• Pile height - 4 to 10 ft
• 10 ft space between piles if equipment access is required, no equipment movement on piles
• 1% slope of pile base for leachate drainage
• Less than 45% slope of pile sides
• Membrane under and/or over pile
• Aeration – mechanical mixing or pressurized or vacuum aeration
• Catalyst and water addition – soaker hoses, surface spray or vacuum communication
• Add catalyst during initial mixing operation
• Leachate Collection – gravity or vacuum extraction
• Leachate Treatment – recirculation or treatment
• Off gas treatment required for volatile petroleum compounds like gasoline, generally not required for diesel